The invention concerns a device and a process for rapid and simple thermographic examination of functional surfaces of forming tools according to the pre-characterizing portion of Patent claims 1 and 11. A device for examination of the thermal load of functional surfaces of forming tools is already known from EP 685297 A1.
The optimization of the forming processes and work tools associated therewith is the focus of intensive research and development activity. Of particular interest is measurement of temperature in the work area during the work process. This allows determination in situ of the loading and the friction condition of the work tool. Ever-increasing requirements of work tools dictate higher loads and require more suitable new materials for these work tools. Conventionally, these new materials are applied to the work tools in the form of coatings as in EP 685297 A1. It has been proposed to integrate one (or more) thin film sensors in a functional or wear protective layer of the work tool for determination of the temperature of the wear surface of a forming or machining work tool. This however requires much technical effort and high costs. Besides this, thin film sensors enable temperature measuring only at one point. The determination of the temperature distribution would here be possible only with a multiplicity of sensors, and would require higher technical investment and costs. Besides this, no high dynamic measurement can be accomplished with the integration of a thin film sensor in a work tool.
The task of the present invention is comprised therein, to provide a device for rapid and simple thermographic examination of functional surfaces in forming tools in situ, which makes possible the determination of the temperature distribution without hindering the work process or the goal of the optimization of the wear and structure relationship of the employed work tool without increasing investment and costs, as well as the development of a process with the same advantages.
With respect to the device to be achieved, the inventive task is preferably inventively solved whereby, that the device for thermographic examination of functional surfaces of forming work tools is designed in such a manner,
that it contains a temperature measurement device,
that the forming tool in the area of the functional surface contains at least one channel,
which respectively exhibits one opening facing the work piece,
and
into which thermal radiation emitted from the functional surface to be examined is conducted,
whereby the thermal radiation conducted through a channel is detectable at least indirectly by the temperature measuring device,
so that the work piece side opening of each channel is provided with a window, which is transparent yo thermal radiation.
The fundamental principle of the inventive device is comprised therein, that the thermographic examination of the functional surface occurs from the work tool internal side, through the one or more provided channels. Thereby there is made possible for the first time unimpeded and uninterrupted measurement of the absolute temperatures and two-dimensional temperature distribution of the work piece outer surface directly in the contact area of the work tool-work piece in situ. Thereby the forming process itself can be examined and optimized with respect to the processing parameters, the work tool geometry and the tribo system (material of work tool, work tool coating, lubricant, outer surface topography). The measurements can be carried out not only in the experimental operation, but rather also in be employed series production for dynamic process regulation or feedback control.
The investiture and the costs of the inventive device are small when compared to the application of a measurement field comprised of a plurality of thin layer sensors. Besides this, such a measurement field would only make possible or permit an approximate determination of the spatial temperature distribution, since the thin layer sensors only measure at points.
Depending upon the size of the opening of the channel on the side of the work piece, this may or may not be provided with a window.
In an advantageous embodiment of the inventive device, the work piece side opening of each channel is provided with a window and upon this window a layer or coating or overlay is applied.
Thereby, the determination of the utilization relationship and in particular the employment limits of various work tool coatings under real application conditions is made possible. If IR transparent coatings are employed, for example diamond or similar coatings or overlays, then the contact area of work tool-work piece can be examined in situ. If IRxe2x80x94opaque coatings are employed, then the temperature distribution is first conveyed from the contact area out via conductive paths through the coating to the underside and only thereafter is further relayed by means of thermal radiation in the direction of the temperature measuring device. Work tool coatings are conventionally only a few micrometers thick, thus the warmth conducted through them falsifies the measurement of the temperature distribution only within an acceptable magnitude.
In a further advantageous embodiment of the inventive device, there is over the window, or in place of the window, a covering secured, which is comprised of a tool material conventionally employed in forming processes, for example a chrome-containing tool steel.
The advantage of this embodiment is comprised in the simple, rapid and economical exchangeability of the covering and therewith the tool material and the tool surface.
In a further advantageous embodiment of the inventive device, a coating is applied to the covering.
The advantage of this embodiment is comprised therein, that the covering is easily exchangeable and independent from the rest of the tool, or the measuring device can be coated. This leads to a time and cost saving and makes possible direct or immediate comparative measurements with a basic device. As a result of the comparative measurements, it becomes possible to develop iteratively in the optimizing increments an optimal coating suited for the use for each application case of the forming technique.
Beyond this, the coatingxe2x80x94of a conventional tool steel employed in forming processesxe2x80x94allows itself to be coated better and with greater bonding strength with the materials interesting for forming techniques than most of the other outer surfaces, which must withstand the loads during a forming process, for example a diamond outer surface, on which because of the covalent bonding character a sufficient adhesion is achieved. An adequate adhesion is, however, a precondition for temperature measurements over longer periods of time during the forming processes, without suffering from a breaking off of the coating. This makes possible durability testing of the forming process with parallel temperature measurements.
The window situated below the coating supports this superficially, and this prevents a deformation or a breaking of a thin covering on the basis of the loading during the forming process. In the use of coverings with sufficient mechanical load resistance, it becomes possible to dispense with the window.
Generally, a calibration of the device, in particular the temperature-measuring device to the emission characteristics of the material or, as the case may be, the coating, is necessary. The calibration expenditure for various materials or tool coatings can, however, be reduced to a single calibration, in which respectively one coating of a single material conventionally employed in the forming processes is applied over or in place of the window on the channel.
In an advantageous embodiment of this device, the material and the thickness of the coating is so selected, that the image of the temperature distribution of the functional surface (or as the case may be, the lower side of the coating), which via thermal conductivity is directed through the covering, is not adulterated perpendicular to this intended image direction beyond a defined base value.
Since the thermal conduction through the covering occurs not only in the desired direction but rather also perpendicular thereto, the temperature distribution, which can be found at the underside of the covering, in comparison to the temperature distribution which can be found in the contact area of the work tool (covering)xe2x80x94work piece or at the lower side of the (covering) coating, is slightly smeared or attenuated.
In order to be able to determine the influence of a particular covering, the temperature distribution must be measured with and without covering. This can easily be carried out with IR transparent coverings. By investigation of coverings of various materials and thicknesses, both values can be so iteratively optimized with respect to a minimal deviation of the measured and the actual temperature distribution in the coating. A covering optimized in this manner can then be employed for the thermographic examination of various coatings.
Experimental results have shown that the deviation of the measured and the actual temperature distribution in the coating is insignificant when the covering consists of a tool steel 160 CrMoV 12 (German material number: 1.2379) and has a thickness of not more than 300 xcexcm.
In a further advantageous embodiment of this device, the covering is secured by adhesion. This method of securing makes possible a surface-wise and therewith more even securing than for example using screws or clamping. During the forming process, the covering is subjected to loads, which in the case of uneven securing has a greater likelihood of leading to internal tensions and thereby lead to falsification of the images transferred by thermal conductivity of the temperature distribution of the functional surfaces or as the case may be, the coating. Beyond this, screw heads or clamp securing means can, on the basis of the small thickness of the coating, hardly be sunk or counter-sunk into them, whereby the possibilities for their positioning are strongly reduced, since they should not influence the forming process.
Suitable adhesives are, for example, conventionally available two-component adhesives, which are transparent for thermal conductivity and which can for short periods tolerate temperature peaks of up to 500xc2x0 C. and which can continuously tolerate temperatures of up to 200xc2x0 C. without critical deterioration of the adhesion.
In a further advantageous embodiment of this device, the window is constructed of a material, which possesses a similar thermal productivity as the covering. Thereby, heat accumulation at the lower side of the covering is avoided, which would falsify the thermographic examination.
Particularly suitable is a diamond or a germanium window. The advantage is comprised therein that these materials on the one hand have a good transparency for thermal radiation and thus are good at conveying the image of the temperature distribution. On the other hand, they possess a good thermal productivity for conducting off the warmth at the lower side of the covering and thus prevent an accumulation of heat.
In a further advantageous embodiment of this device, at least one IR mirror is provided in the channel, for further conveyance of the thermal radiation entering from the work piece side into the channel in the direction of the temperature-measuring device. Many forming tools or benders make possible on the basis of their construction shape and their arrangement during the forming process no straight guide of the channel in their inside. In such cases, one or more IR mirrors make possible the further conveyance of the image of the temperature distribution to the temperature-measuring device.
In an alternative advantageous embodiment of this device, a light guide is provided in the channel for the further conveyance in the direction towards the temperature-measuring device of the thermal radiation entering the channel from the work piece side. It is also possible that the image of the temperature distribution can be conveyed via a light guide through a non-linear channel to the temperature-measuring device.
In a further advantageous embodiment of this device, the temperature-measuring device includes a thermal camera. The advantage thereof is comprised therein, that it makes possible examination of the temperature distribution in a simple and very rapid manner and this both for individual selected point in time (individual image) as well also the development during a longer period of time (sequence of individual images).
In a more special advantageous embodiment of this device, this contains in addition an evaluation device for compensation of IR-losses. Both the window as well as the usually present adhesive layer does not possess a 100% transparency for thermal radiation. This loss factor can be determined independent of the thermographic examination of the forming process, and then during the examination of the forming process (or thereafter) can be compensated for by means of the evaluation unit, such as by computer. The corresponding applies for further measurable and routine (not coincidental) occurring influence parameters.
The task with respect to the process to be achieved for thermographic examination of functional surfaces of forming devices is inventively solved thereby,
that the examination occurs during a forming process by means of a temperature-measuring device and
that for examination in the forming tool in the area of the functional surface, at least one channel is provided, and
this preferably is covered with a thermal radiation transparent window,
which leads to the temperature-measurement device,
that during the examination the image of the thermal distribution established during the forming process in the functional surface is conveyed to the temperature-measuring device through the window and the channel by means of thermal conductivity.
The advantages of the inventive processes are the same as the advantages already enumerated for the inventive device.
In a further advantageous embodiment of this process, a coating is applied to the window prior to testing, and then the channel is covered with the window.
Thereby, the determination of the employment relationships and in particular the employment limits of various tool coatings under real application conditions are made possible. See the above-corresponding discussion of the device.
In a further advantageous embodiment of this device, a covering is secured over the window or in place of the window, comprised of a tool material conventional in the forming processes.
The advantage of this process step is comprised first therein, that in this manner various tool materials and tool surfaces can be examined, and second in its simple, rapid, and economical manner of employment.
In a further advantageous embodiment of this process, a coating is applied to the covering.
The advantage of this process step is comprised first therein, that with one base device the direct comparative measurements of various coverings is made possible. By means of comparative measurements, a function appropriate optimal covering can be developed iteratively in optimizing increments for each case of application of the forming technique.
In an advantageous embodiment of this process, thermal losses occurring in the path from the covering to the temperature-measuring device are compensated for using a computer. IR-losses necessarily occur in this path. This loss factor can be determined independent of the thermographic examination of the forming process and then during the examination of the forming process (or also thereafter) be compensated via a computer program.
An advantageous method for determining and compensating for the loss factor is comprised therein, that thermographic examination of IR-transparent layers are respectively carried out with and without thermal loss ([with and without window] or [with and without covering and window] or [with and without covering without window]), and that on the basis of these examination results an actual compensation curve can be calculated, and that on the basis of this actual curve the thermal losses during the thermographic examinations of various, that is IR-transparent or IR-opaque layers ([with window and without covering] or [with window and with covering] or [with covering and without window]) can be compensated for by computer program.